Methods for the formation of phosphate coats on iron and steel surfaces by means of applying acid solution containing the phosphates, that is, salts of phosphoric acid, with polyvalent metals, as well as oxidants or other accelerator components to accelerate coat formation, have been known for a long time. Dependent upon the type of application, one distinguishes between three common methods: spraying, dipping, and combination spraying-dipping methods. The use of alkaline cleaning and scouring solutions prior to phosphatization, that is, treatment with phosphates, to clean the metal surfaces to be treated of adhering oils and greases, in particular, as well as of other impurities such as metal particles, is also known. Such cleaning solutions usually contain surface-active substances, such as wetting agents, and emulsifiers, as well as so-called builder substances to increase the emulsifying-saponifying and soil carrying capacity. Examples of the latter components include sodium hydroxide, alkali metal carbonates, alkali metal orthophosphates, corresponding condensed phosphates, that is, polyphosphates, such as sodium pyrophosphate or sodium triphosphate, as well as silicates and borates. Furthermore, coat-refining and coat-activating substances have frequently been added to these cleaning and scouring agents, such as, for example, titanium phosphate. A characteristic feature of such solutions is their content of builder substances, which require a free alkalinity in aqueous solution, that is, whose alkalinity exceeds that of secondary orthophosphates, as well as a content of condensed phosphates. In typical concentrations the pH value of these cleaning solutions is between 8.5 and 11.5.
However, use of the above-described cleansers has a number of serious drawbacks with regard to subsequent phosphatization. Usually the material to be treated first passes through a rinsing zone after the cleaning zone and then is introduced into the phosphatizing zone. In spraying and combination spraying-dipping plants, the material coming to the phosphatizing zone initially comes in contact with the so-called pre-spray from the first spray ring of the phosphatizing zone. In this phase, the rinse water solution adhering to the metal surfaces, which is still contaminated with alkaline cleaning solution, is washed off by the phosphatizing solution while the formation of the phosphate coat starts at the same time. Problems arise here insofar as the pH value of the rinse water solution, which is at first in the alkaline range, successively drops on the metal surface to the acid pH value of the phosphatizing solution, passing through pH values in the range of from 6.5 to 4.5. In this slightly acidic pH range, the formation of passivating iron phosphate coats take place, and such coats impair to a considerable extent the desired coat-forming phosphatization. Additional disturbances of the coat formation also result from the use of condensed phosphates in the cleanser, since these represent strong phosphatizing toxins. Such disturbances, which are caused, on the one hand, by the alkalinity of the residual cleanser solutions and, on the other hand, by their content of condensed phosphates, manifest themselves in streaky, spotty, or passivated phosphatizing coats, whose weight fluctuates considerably. Furthermore these inhomogeneous coats provide only inadequate corrosion protection of the metal surface, particularly in connection with subsequently applied organic coats, such as electrodip coats.
The effect of the above-described negative influences of the cleaning solution on the phosphatization is greater as the velocity of flow of the material through the treatment zones is lower and the stay period of the material in the critical pre-spray zone is thus longer. However, for manufacturing reasons the velocity of flow of the material can not be increased indefinitely. The customary cleaning solutions are also extremely disadvantageous in view of the increasingly used methods with water-saving rinsing zones, for example, cascade methods, since in these methods increasing amounts of cleaning solution in the rinse water must be accepted.